Princeton Reverb 75 with odd decay noise/distortion.

[caesparza]

SInce I recapped the amp, the situation improved a lot. I don't know at this point if the sort of vibration I hear is the same of the audio I attached. It's like I'm running in circles with this amp. But the waveform at the output of V1B caught my attention.

I don't see any evidence of having a conductive board here.

So Tom you think, this is the normal behavior for the amp?

Thanks to everyone for the kind answers.
CE

[Helmholtz]

Pin 7 of V1B has a perfect sine wave (it means that there are no problems at the tone stack), but the plate of that triode (pin 7) has the output attached in the image.

I can't make sense of the above.
Pin 7 of V1B is the grid of the second gain stage. Signal is clean. But then you have a distorted signal at pin 7?
Did you mean plate pin 6?

What are cathode, plate and supply voltages of V1B without signal?

Please post correct schematic for the amp.

[caesparza]

Yes Helmholtz, sorry about that. I get the clipped scope image at the output of V1B (Pin 6).

I attached the corresponding schematic and layout.

The voltages are:
161.8VDC (plate)
1.134VDC (cathode)
0 (grid).

[Helmholtz]

What is the supply voltage at point D?

[caesparza]

238.3VDC

[Helmholtz]

Voltages look ok.
At what input signal voltage do you get a clean signal at V1B pin 6?

[caesparza]

Yes Helmholtz. And tested with another tube with the same results. I even lower the triode to a 12AT7, the problem arises at higher volume with a triode with less gain.

I can lower the input till a get a clear and balanced sine wave, but then if I check later on the signal path the problem appears latter, for instance at the grid of the phase inverter.

[caesparza]

12mV Gives a perfect sin wave with the volume maxed out at 10.

[Helmholtz]

Yes Helmholtz. And tested with another tube with the same results. I even lower the triode to a 12AT7, the problem arises at higher volume with a triode with less gain.

I can lower the input till a get a clear and balanced sine wave, but then if I check later on the signal path the problem appears latter, for instance at the grid of the phase inverter.

Voltage numbers, please.
To scope the PI input grid, your scope must be AC coupled.

What clean power output do you get?

[caesparza]

Ok I think there is a problem with the scope. I took my digital one and got the readings feeding 150mVpp to the input and with the volume maxed out. Channel 1 correspond to pin 6 (after the 0.022 cap) and Channel 2 the grid of the PI.


Everything looks fine!!

[Helmholtz]

Signals look normal but way too large for clean with vol at max.
The PI input only needs around 24Vrms or 68Vpp for full clean output.

[Tom Phillips]

...So Tom you think, this is the normal behavior for the amp?...

When the pre-amp is over driven in an otherwise working Fender amp, I have seen asymmetrical wave forms such as the one you attached to post #5. However, the trailing off distortion in your audio clip attached to post #1 is not normal behavior.

[pdf64]

To replicate the trailing off distortion thing, I'd be using a low signal level at the input.

The issue is miles away from clipping.